It's not bad, but I have grown really fond of Grafana's date/time picker and can whip up a clone of that using React-Bootstrap and React Datepicker (https://reactdatepicker.com) in about ten minutes :)
For what it's worth, the Grafana date picker[0] is "just" react-calendar[1]. The time range picker you see on a dashboard uses that and a bunch of other (custom) stuff.
I am assuming that you're judging the behavior by the minimal example in the top 1% of the site. If you scroll further, you will notice how extensible it is to support not only month and date navigation but many other customizations.
I also assumed it was limited in such ways. Mostly because it is extraordinarily rare that anyone bothers to enable those. I'd much, much, much rather have a singular, consistent full UX available to me instead of defaulting to some "simple elegant" artificially stunted version.
Funnily enough I have a pet peeve with Grafana's picker (which is overall quite nice) - I can't copy and paste the whole date range from one window to another in one go. I'm often punching in varies specific datetime ranges across multiple dashboards so this comes up all the time every day. Sure I've added data links for the cases that I use most frequently but I can't do that for everything. The "recently used ranges" (which is a neat feature) doesn't seem to work across dashboards and seems to require a page reload which baffles me a bit.
(I work on grafana frontend)
[0]: https://github.com/grafana/grafana/blob/4b2a9406596ba63a6945...
[1]: https://www.npmjs.com/package/react-calendar